The subject matter of the present invention pertains to means within a cathode-ray tube electromagnetic deflection system for correcting side pincushion distortion and regulating picture width.
As is known to the art, side pincushion distortion in a cathode-ray tube display system is caused by the development of an image on a display screen having a radius of curvature greater than the minimum distance between the screen and the point of deflection of the beam used to describe the image. Such distortion is characterized by a widening of the image in the horizontal direction at the top and bottom relative to the center. A comprehensive discussion of the pincushion effect may be found in A. E. Popodi, "Linearity Correction For Magnetically Deflected Cathode-Ray Tubes," EDN Magazine, January 1964, pages 124-139.
The most common method for correcting side pincushion distortion is to modulate the horizontal deflection signal with a correction signal varying at the vertical deflection rate. In the typical case, a correction signal is generated according to a predetermined mathematical analysis of the geometries involved and then employed to predistort the horizontal deflection signal before it is applied to the deflection system of the cathode-ray tube. A pertinent example of such a known pincushion correction circuit is that disclosed by Battjes et al U.S. Pat. No. 4,039,899, assigned to the assignee of the present invention. Other pertinent examples include those disclosed by Balaban et al U.S. Pat. No. 4,132,927, Deitz U.S. Pat. No. 3,962,602, Worster et al. U.S. Pat. No. 3,916,254, Bellow U.S. Pat. No. 3,825,796, Chapman U.S. Pat. No. 3,781,590, Schwartz, U.S. Pat. No. 3,772,566 and Williams U.S. Pat. No. 3,517,252.
A common disadvantage of known systems for correcting pincushion distortion is their open loop configuration. After the correction signal has been generated and employed to predistort the horizontal deflection signal, the distorted signal is applied directly to the deflection system of the cathode-ray tube. There is no provision for sampling the actual signal produced within the deflection system to be sure that it follows the signal supplied. Such a provision is especially desirable in an environment of modern digital display systems where the requirements for accuracy and resolution are especially high.
Somewhat associated with the problem of pincushion distortion is the ofttimes attendant problem of maintaining a desired picture width independent of changes in the video line rate. Historically, each time a change occurs in the line rate, the image width changes as well and a manual adjustment is necessary to return it to its original dimension. Although previous attempts, most pertinently that disclosed by Infante U.S. Pat. No. 4,129,807, have been made to regulate picture width independent of changes in the rate, none of those attempts have as yet been completely successful. Moreover, no known means has been developed to accomplish simultaneously both side pincushion correction and picture width regulation via the same closed loop circuit.